The present invention concerns radio frequency and microwave network analyzers and pertains particularly to removing effects of adapters present during vector network analyzer calibration.
A radio frequency (RF) network analyzer system consists of a network analyzer. The network analyzer integrates a radio frequency source with built-in couplers for signal separation, a narrow band receiver, a display and a processor.
Measurement calibration is a process that improves measurement accuracy by using error correction arrays during signal processing to compensate for systematic measurement errors. Measurement calibration is also called Cal, accuracy enhancement, and error correction. Measurement errors are classified as random and systematic errors. Random errors, such as noise and connector repeatability are non-repeatable and not correctable by measurement calibration.
Systematic errors, such as directivity, matching, tracking and crosstalk, are the most significant errors in most RF measurements. Systematic errors are repeatable and for the most part correctable, though small residual errors may remain. These systematic errors may drift with time and temperature.
Systematic errors are due to system frequency response, isolation between the signal paths, and mismatch in the test setup. Frequency response errors (transmission and reflection tracking) account for non-ideal gain and delay which are functions of frequency.
Isolation errors result from energy leakage between signal paths. In transmission measurements, this leakage is called crosstalk. In reflection measurements, this leakage is called imperfect directivity. Directivity is the ability of the signal separation devices to separate forward traveling signals from reverse traveling signals.
Mismatch errors arise when a portion of an incident voltage wave undergoes undesired internal reflections. After multiple internal reflections the reflected wave recombines and introduces undesired variations to the unreflected portion of the incident wave. The level of mismatch error is proportional to the port reflection coefficient. The port reflection coefficient is the ratio of the voltage wave reflected from a port over the voltage wave incident to the port. Mismatch errors can arise from interactions of network analyzer source match with the network analyzer load match as well as source match interaction with the input match of the DUT and load match interaction with the output match of the DUT.
FIG. 4 shows a representation 70 of the systematic errors associated with the measurement of the forward s-parameters (s.sub.11 and s.sub.21). The forward systematic error terms are forward directivity (e.sub.00), forward source match (e.sub.11), forward reflection tracking (e.sub.10 e.sub.01), forward crosstalk (e.sub.30), forward load match (e.sub.22), and forward transmission tracking (e.sub.10 e.sub.32).
FIG. 5 shows a representation 80 of the systematic errors associated with the measurement of the reverse s-parameters (s.sub.22, s.sub.12). The reverse systematic error terms are reverse directivity (e.sub.33), reverse source match (e'.sub.22), reverse reflection tracking (e.sub.23 e.sub.32), reverse crosstalk (e.sub.03), reverse load match (e'.sub.11), and reverse transmission tracking (e.sub.23 e.sub.01).
In both FIG. 4 and FIG. 5, the s-parameters s.sub.11a, s.sub.21a, s.sub.12a, and s.sub.22a represent the actual scattering parameters for the DUT.
FIG. 6 shows a combined representation 90 of representation 70 shown in FIG. 4 and representation 80 shown in FIG. 5. For representation 90 shown in FIG. 6, e.sub.11 and e.sub.22 will have two distinct values depending on whether forward or reverse s-parameters are being measured.
A network analyzer generally has several methods of measuring and compensating for the test system errors to varying degrees of accuracy. Each method removes one or more of the systematic errors through vector error correction. Vector error correction is the process characterizing the systematic errors by measuring calibration standards (devices with known s-parameters) then mathematically removing the effects of the systematic errors from subsequent measurements on various DUTs.
One method of calibration involves the use of shorts, opens, loads, and a thru (direct connection of network analyzer test ports). The opens, shorts and loads are used to calibrate each network analyzer port for reflection measurements. The network analyzer measures each standard across a defined frequency band using a pre-defined number of points. The measurement of the opens, shorts and loads are used to solve for directivity, source match and reflection tracking for each port. The thru is then connected and four measurements are made--forward reflection to characterize forward load match, reverse reflection to characterize reverse load match, forward transmission to characterize forward transmission tracking, and reverse transmission to characterize reverse transmission tracking. The accuracy of the load match measurements is enhanced by accounting for the effects of directivity, reflection tracking and source match determined from the preceding reflection calibrations. The accuracy of the transmission tracking measurements is enhanced by accounting for the effects of source match/load match interactions. Forward and reverse crosstalk are characterized by measuring transmission in both directions with loads connected to both test ports. For more information see Doug Rytting, Advances in Microwave Error Correction Techniques, RF & Microwave Measurement Symposium and Exhibition, Hewlett-Packard Company, 1987, pp. see pages 7-11.
When performing calibration, test port connectors are used to connect the network analyzer to the calibration standards. The test port connectors need not be insertable or of the same family. Typically, a separate calibration kit is required for each connector family. If connectors are non-insertable, an adapter removal calibration is often done which requires two separate calibrations. See Doug Rytting, Advances in Microwave Error Correction Techniques, RF & Microwave Measurement Symposium and Exhibition, Hewlett-Packard Company, 1987, pp. see pages 21-24.
In the simplest case, the test port connectors are insertable which means the two test ports will mate directly to each other without the DUT present. In this case, a single calibration kit is required to determine the vector error correction arrays.
If the DUT is non-insertable the two test ports will not mate together. The simplest non-insertable case is when the connectors are of the same family and same sex--for example if both test ports are Type N(f). In this case a single calibration kit will be able to calibrate the system--the complexity of the calibration may vary depending on the desired accuracy and frequency range. Possible methods (in order of increasing accuracy) for performing calibration using connectors of the same family are the defined thru calibration method, the equal length adapters calibration method, the unknown thru calibration method (requires four channel network analyzer configuration), or the adapter removal calibration method. Each of these methods is more fully described below.
A more complicated non-insertable case occurs when the test ports are different families--for example Type N (f) and X-Band waveguide. In this case multiple calibration kits are needed (one for each family) as well as an adapter with the same port configurations as the DUT. The possible calibration methods are the unknown thru calibration method (requires four channel network analyzer) and the adapter removal calibration method. Each of these methods is more fully described below.
Electronic calibration (ecal) can be performed with ecal modules. Ecal modules can be created and characterized with any test port configuration and provide a simple user procedure for calibration--the ecal module is connected and internal switching changes the calibration states without requiring any operator interaction. This method requires separate ecal modules for each DUT test port configuration. In addition, multiple modules may be required for a given configuration depending on the desired frequency range.
The defined thru calibration method modifies the definition of the THRU standard to include the electrical length of the adapter. The adapter mismatch degrades transmission tracking and load match. This method is used with an SOLT (short, open, load, thru) calibration. The short, open, load(s) are measured at each network analyzer port. The adapter is measured as the thru standard.
The equal length adapters calibration method uses two adapters of equal length in conjunction with an SOLT calibration. One adapter is added to port 2 (or 1) that makes the thru connection insertable. After the thru is measured the added adapter is replaced with an adapter of equal length with the desired test port sex and the remaining calibration standards are connected. The second adapter that was added is left connected when the DUT is measured. Transmission tracking and load match are degraded by the difference in mismatch between the two adapters. If the adapters are similar in construction, the difference in mismatch can be less than the mismatch of a single adapter (defined thru).
The unknown thru calibration method is used with an SOLT calibration. Additional measurements possible with the 4 channel network analyzer allow the thru standard to be characterized during the calibration.
The adapter removal calibration method performs two separate two port calibrations. An adapter with the same port configurations as the DUT is used. One calibration is made with the adapter connected to port 2. The next calibration is made with the adapter connected to port 1. The appropriate error terms from both calibrations are selected and modified as needed to create the desired calibration set. This method can use any type of two-port calibration (TRL or SOLT) for each calibration. If the DUT connectors are different connector families, a separate calibration kit is required for each.